1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and, more particularly, to a liquid crystal display apparatus which employs in combination a liquid crystal display device and a phase plate, thereby improving the response characteristics and the viewing angle characteristics of the display screen.
2. Description of the Related Art
A liquid crystal display device using a nematic liquid crystal display device has conventionally been used widely in number segment type display devices such as clocks and desktop calculators. In recent years, such liquid crystal display devices have also been used in word processors, notebook type personal computers, car liquid crystal TVs, and the like.
A liquid crystal display apparatus typically includes a light transmissive substrate on which electrode lines, and the like, are provided for turning pixels ON/OFF. For example, in an active matrix type liquid crystal display apparatus, active elements such as thin film transistors are provided on the substrate, along with the electrode lines, as switching means for selectively driving the pixel electrodes by which voltages are applied through the liquid crystal layer. In a color liquid crystal display apparatus, a color filter layer for providing colors, e.g., RGB, is provided on the substrate.
An appropriate liquid crystal display mode can be selected for use with each of such liquid crystal display devices according to the twist angle of the liquid crystal molecules. For example, an active drive type twisted nematic liquid crystal display mode (hereinafter, xe2x80x9cTN modexe2x80x9d) and a multiplex drive type super twisted nematic liquid crystal display mode (hereinafter, xe2x80x9cSTN modexe2x80x9d) are well known in the art.
In the TN mode, nematic liquid crystal molecules are oriented in a 90xc2x0 twist so as to guide light along the twist, thereby producing a display. The STN mode effectively utilizes the phenomenon that when the twist angle of the nematic liquid crystal molecules is increased to be greater than 90xc2x0, the transmission therethrough changes rapidly for voltages in the vicinity of the threshold voltage applied through the liquid crystal layer.
In the STN mode, the background of the display screen is colored in a unique color due to interference of colors because the STN mode utilizes the birefringence effect of a liquid crystal material. In order to solve such problems so as to produce a black and white display with the STN mode, it is believed to be effective to employ an optical compensator. Display modes employing an optical compensator can be generally classified into the double super twisted nematic phase compensation mode (hereinafter, xe2x80x9cDSTN modexe2x80x9d) and the film type phase compensation mode in which an optically anisotropic film is employed (hereinafter, xe2x80x9cfilm added modexe2x80x9d).
The DSTN mode employs a two-layer structure including a display liquid crystal cell in which the liquid crystal molecules are twisted in a certain direction and another liquid crystal cell in which the liquid crystal molecules are twisted in the opposite direction. The film added mode employs a structure in which an optically anisotropic film is provided. It is believed that the film added mode is advantageous in that it is light in weight and low in cost. Since the black and white display characteristics with the STN modes have been improved by the employment of such phase compensation methods, color STN liquid crystal display apparatuses have been realized in the art in which a color filter layer is provided in an STN mode display apparatus.
On the other hand, the TN modes can be generally classified into the normally black mode and the normally white mode. In the normally black mode, a pair of polarizers are arranged so that their polarization directions are parallel to each other, whereby a black display is produced in the absence of an ON voltage through the liquid crystal layer (i.e., in the OFF state). In the normally white mode, a pair of polarizers are arranged so that their polarization directions are orthogonal to each other, whereby a white display is produced in the OFF state. The normally white mode is advantageous in the terms of the display contrast, the color reproducibility, the viewing angle dependency of the display, etc.
However, a liquid crystal display apparatus of the above-described TN mode has a problem as follows. In a TN mode liquid crystal display apparatus, the liquid crystal molecules have a refractive index anisotropy xcex94n and the liquid crystal molecules are in an inclined orientation with respect to the upper and lower substrates. As a result, the viewing angle dependency may be substantial such that the contrast of the displayed image substantially varies depending upon the direction and the angle from which the display is viewed by the observer.
FIG. 34 schematically illustrates in a cross-sectional view the structure of a TN liquid crystal display device. In the state as shown, a gray-level voltage is being applied through the liquid crystal layer, whereby the liquid crystal molecules are slightly raised. With the TN liquid crystal display device in this state, linearly-polarized light travelling in the direction normal to the surfaces of the pair of substrates (the xe2x80x9cnormal directionxe2x80x9d) passes through the liquid crystal molecules at an angle which is different from the angle at which linearly-polarized light travelling in a direction inclined with respect to the normal direction passes through the liquid crystal molecules. Since the liquid crystal molecules have the refractive index anisotropy xcex94n, passage of the linearly-polarized light from the different directions produces normal light and abnormal light with a phase difference therebetween. Due to such a phase difference, the incident light is converted into elliptically-polarized light, thereby causing the viewing angle dependency.
Within a liquid crystal layer in an actual liquid crystal display device, liquid crystal molecules in the vicinity of the midpoint between the substrates have a tilt angle which is different from that of other liquid crystal molecules on or near the substrate, and the liquid crystal molecules have a 90xc2x0 twist between the substrates about the normal direction.
Thus, the linearly-polarized light passing through the liquid crystal layer is subject to various degrees of birefringent effect depending upon its direction and angle, thereby exhibiting a complicated viewing angle dependency.
This viewing angle dependency causes phenomena such as the following: the display screen is colored when the viewing angle is inclined from the direction normal to the screen toward the normal viewing direction, i.e., toward the lower side of the screen by a certain angle or more (hereinafter, the xe2x80x9ccoloring phenomenonxe2x80x9d); and the black and the white are reversed (hereinafter, the xe2x80x9cblack and white reversal phenomenonxe2x80x9d). When the viewing angle is inclined toward the counter-normal viewing direction, i.e., toward the upper side of the screen, the contrast is reduced rapidly.
The above-described liquid crystal display apparatus has another problem follows. The larger the display screen, the narrower is the viewing angle. When a large liquid crystal display screen is viewed by the observer from the normal direction within a short distance from the screen, the display color in an upper portion of the screen may be different from the display color in a lower portion of the screen due to an influence of the viewing angle dependency. This is because the observer being so close to the screen, although in the normal direction makes the situation substantially the same as that where a small display screen is viewed from an inclined direction.
In order to address the problems associated with the viewing angle dependency, it has been proposed in the art to insert a phase plate (a phase film) as an optionally anisotropic optical element between a liquid crystal display device and one of a pair of polarizers (see, for example, Japanese Laid-Open Publication No. 5-313159).
According to this method, a phase plate is provided on one side or on both sides of the liquid crystal layer having a refractive index anisotropy. Linearly-polarized light, which has passed through the liquid crystal layer having the refractive index anisotropy and thus have been converted into elliptically-polarized light, passes through the phase plate so as to compensate for the change in the phase difference between the normal light and the abnormal light occurring within the viewing angle, thereby re-converting the elliptically-polarized light into linearly-polarized light. Thus, the viewing angle dependency is improved.
However, even with the use of the phase plate, the reversal phenomenon in a normal viewing direction and the reduction in the contrast in a counter-normal viewing direction cannot sufficiently be improved at the same time.
In view of this, Japanese Laid-Open Publication No. 6-75116 proposes a method which employs a phase plate in which a principal refractive index direction of the index ellipsoid is inclined with respect to the direction normal to the surface of the phase plate. In this method, the following two types of phase plates are used.
One of the two types of phase plates is as follows. The direction of the minimum principal refractive index of the three principal refractive indices of the index ellipsoid is parallel to the surface of the phase plate. The direction of one of the remaining two principal refractive indices is inclined at angle xcex8 with respect to the surface of the phase plate, with the direction of the other principal refractive index being inclined at angle xcex8 with respect to the direction normal to the surface of the phase plate. The value of xcex8 satisfies the relationship of 20xc2x0xe2x89xa6xcex8xe2x89xa670xc2x0.
The other one of the two types of phase plates is as follows. The three principal refractive indices na, nb, nc of the index ellipsoid satisfy the relationship of na=nc greater than nb. The direction of the principal refractive index nb, which is parallel to the direction normal to the surface of the phase plate and is orthogonal to one of the in-plane principal refractive index nc (or na), is inclined clockwise or counterclockwise from its original direction about an axis along the direction of one of the in-plane principal refractive indices na (or nc).
The former of the above-described two types of phase plates may be either uniaxial or biaxial. A phase plate of the second type may be either a single phase plate or a combination of two such phase plates in which the direction of the principal refractive index nb of one phase plate is at an angle of 90xc2x0 with respect to the direction of the principal refractive index nb of the other phase plate.
In a liquid crystal display apparatus in which at least one phase plate is provided between the liquid crystal display device and the polarizer, as described above, the change in the contrast, the coloring phenomenon and the reversal phenomenon occurring in the displayed image depending upon the viewing angle can be improved to a certain degree.
However, both of the TN mode and the STN mode have a problem in that the response speed of the liquid crystal material is low. Typically, the response time is about 30 ms in the TN mode and about 100 ms in the STN mode. Normally, an image signal is updated with a frequency of 60 Hz, meaning that a new image is displayed for each frame period, i.e., for every 16.7 ms. Thus, with the liquid crystal mode with such a low response speed, the liquid crystal material cannot completely respond to an image signal within one frame period. In fact, existing liquid crystal display apparatuses have a ghost image problem in a motion picture display, which represents a significant reduction in the display quality.
A homogeneous orientation mode is well known in the art as a liquid crystal mode which has a shorter response time than those of the TN and STN modes. In this mode, the respective alignment films on two glass substrates have opposite and parallel (antiparallel) rubbing directions with respect to each other, and no chiral agent is added to the liquid crystal material. As a result, there is no twist of the liquid crystal molecules as those in the TN mode or the STN mode. It is believed that the response time of the homogeneous orientation mode is shorter than that of a twist mode because of the simple orientation. In fact, the homogeneous orientation mode gives a response speed which is about one half or less of that of the TN mode.
However, the homogeneous orientation mode also has some problem as follows.
First, with a structure as shown in FIG. 35, a voltage-transmission curve as shown in FIG. 36 is obtained, which indicates that a sufficient black display is not obtained for a voltage range over a few volts. In order to solve this problem, a phase plate of a uniaxially-drawn film has been employed in the art, as shown in FIG. 37. Thus, a voltage-transmission curve as shown in FIG. 38 is obtained, thereby realizing a black display over a greater voltage range.
However, as shown in FIG. 39, the viewing angle characteristics of such a display device are not desirable. While it is a well-known technique in the art to improve the viewing angle characteristics by adding a negative phase plate 301 having a negative refractive index anisotropy, as shown in FIG. 40, the improvement in the viewing angle characteristics may be insufficient as can be seen from FIG. 41.
It is known in the art that a negative inclined phase plate is widely employed in a TN panel for viewing angle compensation purposes (U.S. Pat. No. 5,506,706, Yamahara Patent of Sharp Co.). However, it has been believed in the art that it is not generally preferable to employ a negative inclined phase plate in combination with an ECB mode (e.g., the homogeneous orientation mode, the STN orientation mode, etc.) for the following reason.
Where an inclined phase plate is employed, the design optimization is limited by the following three parameters: the in-plane retardation; the retardation along the thickness direction; and the inclination angle of the symmetry axis of the index ellipsoid (the nz direction in the present invention) included in the phase plate with respect to the film surface. In principle, the inclined phase plate can be used with many liquid crystal modes by appropriately setting the three parameters.
In an actual production process, however, once two of the three parameters, e.g., the in-plane retardation and the inclination angle, are fixed, the retardation along the thickness direction is automatically determined. Thus, it is impossible to optimize all of the parameters. It may be possible to avoid this problem. However, it would be then necessary to change not only the thickness of the film but also the material of the phase plate itself each time the liquid crystal mode or the panel gap is changed. Thus, in view of the actual production process, it has been impractical to employ a phase plate in which the three parameters are optimized.
Recently, this technique has been more widely used in the TN liquid crystal mode for the following reason. Since the rubbing directions on the upper and lower substrates are orthogonal to each other, the in-plane remaining retardation of the liquid crystal layer becomes virtually zero (i.e., no need for compensation) when the voltage applied through the liquid crystal layer exceeds a certain level (which is in many cases set to 4 to 5 V). Therefore, the retardation along the thickness direction and the inclination angle can be utilized by employing an upper and lower film each having an arbitrary in-plane retardation so that their rubbing directions are orthogonal to each other.
According to one aspect of this invention, there is providing a liquid crystal display apparatus, including: a liquid crystal display device, the liquid crystal display device including: a pair of light transmissive substrates; a transparent electrode layer and an alignment film which are provided on one surface of each of the light transmissive substrates, the surface facing the other one of the light transmissive substrates; and a liquid crystal layer interposed between the pair of light transmissive substrates, wherein: the liquid crystal layer includes liquid crystal molecules; the liquid crystal molecules on a surface of each of the alignment films are pre-tilted in the same direction and by the same angle as those on a surface of the other one of the alignment films; and the liquid crystal layer has a homogeneous orientation; a pair of polarizers provided on respective sides of the liquid crystal display device; and at least one inclined phase plate including an index ellipsoid having three principal refractive indices na, nb and nc, wherein: the three principal refractive indices na, nb and nc of the index ellipsoid satisfy a relationship of na=nb greater than nc; each of the principal refractive indices na and nb is a principal refractive index along a direction in a surface of the inclined phase plate, and the principal refractive index nc is a principal refractive index along a direction normal to the surface of the inclined phase plate; the index ellipsoid is inclined by inclining the direction of the principal refractive index nc and the direction of one of the in-plane principal refractive indices na and nb with respect to each other about an axis extending along the direction of the other one of the in-plane principal refractive indices na and nb; the inclined phase plate is provided between the liquid crystal display device and at least one of the polarizers; and the inclined phase plate is arranged so that an inclination direction of the index ellipsoid as being projected onto the surface of one of the light transmissive substrates is generally parallel or antiparallel to an orientation direction of the liquid crystal molecules.
In one embodiment of the invention, the liquid crystal display apparatus further includes at least one negative phase plate including an index ellipsoid having three principal refractive indices nx, ny and nz along an x axis, a y axis and a z axis, respectively, the three principal refractive indices nx, ny and nz satisfy a relationship of nx=ny greater than nz; the x axis and the y axis extend in the surface of the negative phase plate and the z axis extends in a direction normal to the surface of the negative phase plate; and the negative phase plate is provided between the liquid crystal display device and at least one of the polarizers.
In one embodiment of the invention, the liquid crystal display apparatus further includes at least one positive phase plate including an index ellipsoid having three principal refractive indices nx, ny and nz, the principal refractive indices nx and ny being along an x axis and a y axis, respectively; the principal refractive indices nx and ny satisfy a relationship of nx greater than ny; the x axis and the y axis extend in the surface of the positive phase plate; the positive phase plate is provided between the liquid crystal display device and at least one of the polarizers; each of the polarizers has an absorption axis; and the positive phase plate is arranged so that the y axis substantially coincides with the absorption axis.
In one embodiment of the invention, the liquid crystal display apparatus further includes at least one negative phase plate including a negative index ellipsoid having three principal refractive indices nx1, ny1 and nz1 along an x1 axis, a y1 axis and a z1 axis, respectively; the three principal refractive indices nx1, ny1 and nz1 satisfy a relationship of nx1=ny1 greater than nx1; the x1 axis and the y1 axis extend in the surface of the negative phase plate and the z1 axis extends in a direction normal to the surface of the negative phase plate; the negative phase plate is provided between the liquid crystal display device and at least one of the polarizers; the liquid crystal display apparatus further includes at least one positive phase plate including a positive index ellipsoid having three principal refractive indices nx2, ny2 and nz2 along an x2 axis, a y2 axis and a x2 axis, respectively; the principal refractive indices nx2 and ny2 satisfy a relationship of nx2 greater than ny2; the x2 axis and the y2 axis extend in the surface of the positive phase plate; the positive phase plate is provided between the liquid crystal display device and at least one of the polarizers; each of the polarizers has an absorption axis; and the positive phase plate is arranged so that the y2 axis substantially coincides with the absorption axis.
In one embodiment of the invention, the inclined phase plate is arranged so that the inclination direction of the index ellipsoid and a pre-tilt direction of the liquid crystal molecules are substantially opposite to each other.
According to still another aspect of this invention, there is provided a liquid crystal display apparatus, including: a liquid crystal display device, the liquid crystal display device including: a pair of light transmissive substrates; a transparent electrode layer and an alignment film which are provided on one surface of each of the light transmissive substrates, the surface facing the other one of the light transmissive substrates; and a liquid crystal layer interposed between the pair of light transmissive substrates, wherein: the liquid crystal layer includes liquid crystal molecules; the liquid crystal molecules on a surface of each of the alignment films are pre-tilted in the same direction and by the same angle as those on a surface of the other one of the alignment films; the liquid crystal layer has a homogeneous orientation; a pair of polarizers provided on respective sides of the liquid crystal display device; and at least one inclined phase plate including an index ellipsoid having three principal refractive indices na, nb and nc, wherein: the three principal refractive indices na, nb and nc of the index ellipsoid satisfy a relationship of na=nb greater than nc; each of the principal refractive indices na and nb is a principal refractive index along a direction in a surface of the inclined phase plate, and the principal refractive index nc is a principal refractive index along a direction normal to the surface of the inclined phase plate; the index ellipsoid is inclined by inclining the direction of the principal refractive index nc and the direction of one of the in-plane principal refractive indices na and nb with respect to each other about an axis extending along the direction of the other one of the in-plane principal refractive indices na and nb; the inclined phase plate is provided between the liquid crystal display device and at least one of the polarizers; the liquid crystal display apparatus further includes at least one positive phase plate including an index ellipsoid having three principal refractive indices nx, ny and nz, the principal refractive indices nx and ny being along an x axis and a y axis, respectively; the principal refractive indices nx and ny satisfy a relationship of nx greater than ny; the x axis and the y axis extend in the surface of the positive phase plate; the positive phase plate is provided between the liquid crystal display device and at least one of the polarizers; each of the polarizers has an absorption axis; the positive phase plate is arranged so that the y axis substantially coincides with an orientation direction of the liquid crystal molecules; and the inclined phase plate is arranged so that an inclination direction of the index ellipsoid as being projected onto the surface of one of the light transmissive substrates is generally parallel or antiparallel to an orientation direction of the liquid crystal molecules.
In one embodiment of the invention, the liquid crystal display apparatus further includes at least one negative phase plate including a negative index ellipsoid having three principal refractive indices nx1, ny1 and nz1 along an x1 axis, a y1 axis and a z1 axis, respectively; the three principal refractive indices nx1, ny1 and nz1 satisfy a relationship of nx1=ny1 greater than nz1; the x1 axis and the y1 axis extend in the surface of the negative phase plate and the z1 axis extends in a direction normal to the surface of the negative phase plate; and the negative phase plate is provided between the liquid crystal display device and at least one of the polarizers.
In one embodiment of the invention, an inclination angle of the index ellipsoid is equal to or greater than 10xc2x0 and less than or equal to 80xc2x0.
In one embodiment of the invention, the inclination angle of the index ellipsoid is equal to or greater than 20xc2x0 and less than or equal to 50xc2x0.
In one embodiment of the invention, where the inclined phase plate is provided on both sides of the liquid crystal display device, a product (naxe2x88x92nc)xc3x97d of a difference between the principal refractive index na and the principal refractive index nc and a thickness d of the inclined phase plate is set to be in a range from 15 nm to 700 nm; and where the inclined phase plate is provided only on one side of the liquid crystal display device, the product (naxe2x88x92nc)xc3x97d is set to be in a range from 30 nm to 1500 nm.
In one embodiment of the invention, where the inclined phase plate is provided on both sides of the liquid crystal display device, the product (naxe2x88x92nc)xc3x97d is set to be in a range from 33 nm to 159 nm; and where the inclined phase plate is provided only on one side of the liquid crystal display device, the product (naxe2x88x92nc)xc3x97d is set to be in a range from 66 nm to 318 nm.
In one embodiment of the invention, where the inclined phase plate is provided on both sides of the liquid crystal display device, a product (naxe2x88x92nc)xc3x97d of a difference between the principal refractive index na and the principal refractive index nc and a thickness d of the inclined phase plate is set to be in a range from 1 nm to 200 nm; and where the inclined phase plate is provided only on one side of the liquid crystal display device, the produce (naxe2x88x92nc)xc3x97d is set to be in a range from 2 nm to 400 nm.
In one embodiment of the invention, where the inclined phase plate is provided on both sides of the liquid crystal display device, the product (naxe2x88x92nc)xc3x97d is set to be in a range from 30 nm to 150 nm; and where the inclined phase plate is provided only on one side of the liquid crystal display device, the product (naxe2x88x92nc)xc3x97d is set to be in a range from 60 nm to 300 nm.
In one embodiment of the invention, each of the polarizers has an absorption axis; and an angle between a direction of the absorption axis and an inclination direction of the index ellipsoid in the inclined phase plate is greater than xe2x88x925xc2x0 and less than 50xc2x0.
In one embodiment of the invention, an angle between an orientation direction of the liquid crystal molecules and an inclination direction of the index ellipsoid in the inclined phase plate is greater than xe2x88x925xc2x0 and less than 5xc2x0.
In one embodiment of the invention, an angle between an orientation direction of the liquid crystal molecules and an inclination direction of the index ellipsoid in the inclined phase plate is 0xc2x0.
In one embodiment of the invention, an angle between an orientation direction of the liquid crystal molecules and an inclination direction of the index ellipsoid in the inclined phase plate is greater than 40xc2x0 and less than 50xc2x0.
In one embodiment of the invention, an angle between an orientation direction of the liquid crystal molecules and an inclination direction of the index ellipsoid in the inclined phase plate is 45xc2x0.
In one embodiment of the invention, where the negative phase plate is provided on both sides of the liquid crystal display device, a product (nxxe2x88x92nz)xc3x97d of a difference between the principal refractive index nx and the principal refractive index nz and a thickness d of the negative inclined phase plate is set to be in a range from 5 nm to 200 nm; and where the negative inclined phase plate is provided only on one side of the liquid crystal display device, the product (nxxe2x88x92nz)xc3x97d is set to be in a range from 10 nm to 400 nm.
In one embodiment of the invention, where the negative phase plate is provided on both sides of the liquid crystal display device, the produce (nxxe2x88x92nz)xc3x97d is set to be in a range from 35 nm to 105 nm; and where the negative inclined phase plate is provided only on one side of the liquid crystal display device, the product (nxxe2x88x92nz)xc3x97d is set to be in a range from 70 nm to 210 nm.
In one embodiment of the invention, where the negative phase plate is provided on both sides of the liquid crystal display device, a product (nxxe2x88x92nz)xc3x97d of a difference between the principal refractive index nx and the principal refractive index nz and a thickness d of the negative inclined phase plate is set to be in a range from 1 nm to 100 nm; and where the negative inclined phase plate is provided only on one side of the liquid crystal display device, the product (nxxe2x88x92nz)xc3x97d is set to be in a range from 2 nm to 200 nm.
In one embodiment of the invention, where the negative phase plate is provided on both sides of the liquid crystal display device, the product (nxxe2x88x92nz)xc3x97d is set to be in a range from 1 nm to 30 nm; and where the negative inclined phase plate is provided only on one side of the liquid crystal display device, the product (nxxe2x88x92nz)xc3x97d is set to be in a range from 2 nm to 60 nm.
In one embodiment of the invention, where the positive phase plate is provided on both sides of the liquid crystal display device, a product (nxxe2x88x92ny)xc3x97d of a difference between the principal refractive index nx and the principal refractive index ny and a thickness d of the positive inclined phase plate is set to be in a range from 1 nm to 125 nm; and where the positive inclined phase plate is provided only on one side of the liquid crystal display device, the product (nxxe2x88x92ny)xc3x97d is set to be in a range from 2 nm to 250 nm.
In one embodiment of the invention, where the positive phase plate is provided on both sides of the liquid crystal display device, the product (nxxe2x88x92ny)xc3x97d is set to be in a range from 30 nm to 90 nm; and where the positive inclined phase plate is provided only on one side of the liquid crystal display device, the product (nxxe2x88x92ny)xc3x97d is set to be in a range from 60 nm to 180 nm.
In one embodiment of the invention, where the positive phase plate is provided on both sides of the liquid crystal display device, a product (nxxe2x88x92ny)xc3x97d of a difference between the principal refractive index nx and the principal refractive index ny and a thickness d of the positive inclined phase plate is set to be in a range from 1 nm to 100 nm; and where the positive inclined phase plate is provided only on one side of the liquid crystal display device, the product (nxxe2x88x92ny)xc3x97d is set to be in a range from 2 nm to 200 nm.
In one embodiment of the invention, where the positive phase plate is provided on both sides of the liquid crystal display device, the product (nxxe2x88x92ny)xc3x97d is set to be in a range from 5 nm to 40 nm; and where the positive inclined phase plate is provided only on one side of the liquid crystal display device, the product (nxxe2x88x92ny)xc3x97d is set to be in a range from 10 nm to 80 nm.
In one embodiment of the invention, the inclined phase plate includes a support made of a transparent organic polymer and a discotic liquid crystal material cured on the support in an inclined orientation.
In one embodiment of the invention, the inclined phase plate includes a support made of a transparent organic polymer and a discotic liquid crystal material cured on the support in a hybrid orientation.
In one embodiment of the invention, a product of a thickness of the liquid crystal layer and a refractive index anisotropy (xcex94n) of the liquid crystal layer is in a range from 180 nm to 500 nm.
In one embodiment of the invention, the product of the thickness of the liquid crystal layer and the refractive index anisotropy (xcex94a) of the liquid crystal layer is in a range from 220 nm to 350 nm.
In one embodiment of the invention, the liquid crystal display apparatus lumber includes an element for diffusing transmitted light in normal and near-normal viewing directions toward upper and loner directions.
In one embodiment of the invention, the liquid crystal display apparatus further comprises at least one positive phase plate including an index ellipsoid having three principal refractive indices nx, ny and nz, the principal refractive indices nx and ny being along an x axis and a y axis, respectively the principal refractive indices nx and ny satisfy a relationship of nx greater than ny; the x axis and the y axis exist is a surface of the positive phase plate; the positive phase plate is provided between the inclined phase plate and the liquid crystal display device; each of the polarizers has an absorption axis; the x axis, which is a slog axis of the positive phase plate, is substantially parallel or substantially orthogonal to an inclination direction of the index ellipsoid of the inclined phase plate as being projected onto the surface of one of the light transmissive substrates; and an angle between the absorption axle of each of the polarizers and the slow axle of the positive phase plate is substantially 45xc2x0.
In one embodiment of the invention, the liquid crystal display apparatus further comprises a positive phase plate including an index ellipsoid having three principal refractive indices nx, ny and nz the principal refractive indices nx and ny being along an x axis and a y axis, respectively; the principal refractive indices nx and ny satisfy a relationship of nx greater than ny; the x axis and the y axis exist in a surface of the positive phase plate; the positive phase plate is provided between the inclined phase plate and at least one of the polarizers; each of the polarizers has an absorption axis; the x axis, which is a slow axis of the positive phase plate, is substantially parallel or substantially orthogonal to an inclination direction of the index ellipsoid of the inclined phase plate as being projected onto the surface of one of the light transmissive substrates; and an angle between the absorption axis of each of the polarizers and the slow axis of the positive phase plate is substantially 45xc2x0.
In one embodiment of the invention, a difference between (1) a sum of an in-plane retardation of the negative inclined phase plate and an in-plane retardation of the positive inclined phase plate and (2) a retardation of the liquid crystal layer when a black display is produced by the liquid crystal display apparatus is within xc2x110 nm.
In one embodiment of the invention, the sum of the in-plane retardation of the negative inclined phase plate and the in-plane retardation of the positive inclined phase plate is less than or equal to 100 nm.
With the above-described structure, even when linearly-polarised light passes through the birefringent liquid crystal layer thus producing normal light and abnormal light and converting the linearly-polarised light into elliptically-polarised light due to the phase difference therebetween such elliptically-polarised light can be compensated for by the use of the optical phase plate in which the short axis of the index ellipsoid including the principal refractive index nc is inclined with respect to the direction normal to the surface of the optical phase plate.
Moreover, a liquid crystal display device having a homogeneous orientation may be provided with a negative phase plate where the direction along which the index ellipsoid of the phase plate is inclined is opposite to the pro-tilt angle of the liquid crystal molecules. In such a case, it is possible to compensate for the optical anisotropy of the liquid crystal molecules a gray-level display or a blank display even when the viewing angle is greatly inclined.
Moreover, a liquid crystal display device having a homogeneous orientation may be provided with a positive phase plate where the direction along which the index ellipsoid of the phase plate is inclined is opposite to the pre-tilt angle of the liquid crystal molecules. In such a case, it is possible to compensate for the optical anisotropy of the liquid crystal molecules in a gray-level display or a blank display oven when the viewing angle is greatly inclined.
Moreover, a liquid crystal display device having a homogeneous orientation may be provided with a positive phase plate and a negative phase plate where the direction along which the index ellipsoid of the phase plate is inclined is opposite to the pre-tilt angle of the liquid crystal molecules. In such a case, it is possible to compensate for the optical anisotropy of the liquid crystal molecules in a gray-level display or a black display even when the viewing angle is greatly inclined. The positive phase plate and the negative phase plate can be replaced with a single biaxial phase plate which has both the characteristics of the positive phase plate and those of the negative phase plate.
Moreover, a liquid crystal display device having a homogeneous orientation may be provided with a phase plate where the direction along which the index ellipsoid of the phase plots is inclined is opposite to the pre-tilt angle of the liquid crystal molecules. In such a case, it is possible to compensate for the optical anisotropy of the liquid crystal molecules in a gray-level display or a bleak display even when the viewing angle is inclined. While the optical phase plate is preferably provided on both sides of the liquid crystal display device, the optical phase plate may alternatively be provided only on one side of the liquid crystal display device, and the improvement of the viewing angle characteristics can still be obtained.
Moreover, the normal-viewing-direction display characteristics of the liquid crystal display devise having a homogeneous orientation can be optically compensated for by the use of a positive phase plate, while the inclined-viewing-direction display characteristics can be optically compensated for by the use of an inclined phase plate in which the index ellipsoid is inclined. Thus, it is possible to obtain a liquid crystal display apparatus having a wide viewing angle.
In a liquid crystal display apparatus of the present invention, the optical compensation for the liquid crystal molecules in the vicinity of the glass substrate is provided by the inclined phase plate in which the index ellipsoid is inclined. In another liquid crystal display apparatus of the present invention, the optical compensation for the liquid crystal molecules in the middle of the liquid crystal layer along the thickness direction thereof is further provided by the negative phase plate. Thus, it is possible to compensate for the optical anisotropy of the liquid crystal molecules in a gray-level display or a black display even when the viewing angle is greatly inclined.
The two plates, i.e., the inclined phase plate in which the index ellipsoid is inclined and the negative phase plate, can be replaced with a single biaxial or hybrid-orientation phase plate which is obtained by combining together the optical characteristics of such two plates.
The effect of combining a positive phase plate where nx less than ny with a negative inclined phase plate will be described. As already discussed, an inclined phase plate has three parameters, and each of the three parameters is represented by a function of the other two parameters. Thus, except for the TNT mode, optimizing the three parameters at the same time has been practically difficult as it imposes undue burden on the material selection end the deposition process.
As discussed above, we have found the optimal parameters within the currently available materials and processes. However, the design of a liquid crystal panel greatly depends upon its application, and these optimal parameters may not always be realized. Our further study has revealed that if one stresses on the improvement in the viewing angle characteristics, it is important to adjust the retardation along the thickness direction in accordance with the inclination angle, and the in-plane retardation has only a small effect on the viewing angle characteristics.
It has also been discovered that the contribution of the in-plane retardation is mostly to the remaining in-plane retardation compensation in the liquid crystal layer in a black display, and the in-plane retardation has substantially no contribution to the viewing angle characteristics. When the in-plane retardation is the liquid crystal layer in a black display does not exceed 100 nm, a practically sufficient contrast can be obtained for inclined viewing angles.
Therefore, the difference between the in-plane retardation of the liquid crystal layer and that of the inclined phase plate can be compensated for by providing a positive phase plate so that the slow axis is parallel or orthogonal to the rubbing direction of the liquid crystal layer. In such a ease, the improved viewing angle characteristics are comparable to those which are obtained only with an optimal inclined phase plate, and it is possible to provide, by the use of an appropriate positive phase plate, a desirable normal-viewing-direction contrast irrespective of the in-plane retardation of the inclined phase plate. It the sum of the respective in-plane retardation values of the liquid crystal layer, the inclined phase plate and the positive phase plate can be set to be less than or equal to xc2x110 nm, it is possible to obtain a practically sufficient contrast of 200 or more.
According to the present invention the positive phase plate is designed such that nx less than ny. In practice however, the positive phase plate may be designed such that nx greater than ny as the object is to compensate for the retardation of the liquid crystal layer. In an actual design of an inclined phase plots, the production of the phase plate becomes more difficult as the inclination angle is increased, and it is often the case that the in-plane retardation is insufficient for the liquid crystal layer used. In view of this, nx less than ny is employed is the present invention so that the design can be generally employed for various applications.
The positive phase plate can be arranged in any of various ways. For example, positive phase plate may be provided on one or both aides of the liquid crystal layer, or between the inclined phase plate and the liquid crystal layer or between the inclined phase plate and the polarizer. In practice, the positive phase plate would effectively function wherever it is placed. In view of obtained viewing angle characteristics which are more symmetrical and with which images are more easily viewed sad in view of the productivity of the panel, it is preferable to arrange two positive phase plates having substantially the same retardation on the respective sides of the liquid crystal layer. In each of the above-described arrangements, the liquid crystal layer has a homogeneous orientation which is obtained by combining two substrates together so that their rubbing directions are antiparallel to each other. The employment of such a homogeneous orientation, which has no twist in the liquid crystal orientation, results in an improvement, typically about a 2-fold increase, in the response speed to an applied voltage over those obtained by the TN mode which is widely employed in conventional liquid crystal display apparatuses. As a rebuilt the display quality in a high-speed motion picture display is improved over those of conventional liquid crystal display apparatuses.
The present invention described above provides an effect of sufficiently improving the viewing angle over those of the liquid crystal display apparatuses.
However, while the wide viewing angle characteristics obtained by the present invention are substantially symmetrical for left and right viewing angles, the viewing angle characteristics are not symmetrical for lower and upper viewing angles. In view of this, the above-described lens element is provided so as to obtain viewing angle characteristics with a high degree of cylindrical symmetry. Thus, the present invention provides a liquid crystal display apparatus which has substantially no drawbacks as compared to a CRT, i.e., the mainstream display apparatus currently need is television displays.
Thus, the invention described herein makes possible the advantage of providing a liquid crystal display apparatus having a wide viewing angle, a high display quality and a high response speed by employing a homogeneous orientation liquid crystal display device with a high response speed in combination with a phase plate of a special type and structure different from the conventional phase plates go as to improve the viewing angle dependency.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.